The present invention relates to an apparatus for treating medium consistency pulp in connection with different pulp treatment devices or apparatuses. Washers used for washing pulp are disclosed below as an example.
Several types of washing apparatuses are known from the prior art. Known arrangements include diffusers, drum washers/disc washers and Fourdrinier washers, which clearly differ from each other. Pulp is fed into diffuser washers at a consistency of 10%. The feed consistency for drum washers and Fourdrinier washers is normally between 1 and 3%. Drum washers presently used are, for example, suction washers, wash presses and pressure washers.
A conventional suction washer includes a wire coated drum rotatable in a vat or drum. The casing of the drum includes collecting compartments beneath a perforated plate, which each communicate via their own pipe with the valve system on the shaft at the end of the drum. The filtrate is led from the valve through the drop leg to the filtrate chest. Due to the valve construction the suction effect of the drop leg may be arranged at different positions of the web formation.
Web formation in a suction washer is carried out by arranging--by means of a drop leg--reduced pressure inside the drum rotating in the vat, which reduced pressure draws pulp suspension from the vat and against the drum. The fibers of the pulp thicken on the surface of the drum when the liquid penetrates the drum. The consistency of the fiber suspension in the vat is about 0.5-2%, and the consistency of the pulp layer thickened on the drum is about 10-12%. The web formation zone, in other words the part of the rim of the drum, which in the vat is covered by fiber suspension, is about 140.degree.. The maximum rotational speed of the drum is 2 to 2.5 r/min. If the rotational speed is higher the collecting compartments and pipes of the filtrate are not able to empty.
Washing is carried out as a displacement wash by showering washing liquid on the surface of the drum protruding from the vat, which due to the reduced pressure is absorbed through the pulp layer and displaces a majority of the chemical liquid. The width of the displacement zone is approximately 120.degree.. The typical specific square capacity of the suction washer is about 5 BDMT/m.sup.2 /d, wherein the thickness of the pulp web is about 25 mm. In bleaching, the square capacity of the suction washer is about 8 BDMT/m.sup.2 /d and the thickness of the web is about 30 mm. A washer press comprises a drum with a wire coated or drilled perforated plate casing. The pulp feed is carried out at a consistency of 3 to 4% and the knots, unbeaten particles and respective undesired parts are to be discharged from the pulp prior to the washer. There are compartments on the casing of the drum, from which the filtrate is led out via a chamber at the end rim. The drum may also be open so as to gather the filtrate in the drum and let it flow out through the opening at the end.
The length of the web formation stage is about 90.degree. and that of the displacement stage about 150.degree.. The rotational speed of the drum is about 2 r/min and the specific square capacity about 15 to 20 BDMT/m.sup.2 /d. The consistency of the washed pulp may rise even to 30%, when a press roll is used. The displacement, however, takes place at the consistency of 10%, the thickness of the pulp web being about 50 mm.
As an example of a pressure washer there may be mentioned an apparatus according to Finnish patent publication 71961, which mainly comprises a drilled perforated plate drum having 15 to 20 mm high moldings attached on the surface at the distance of about 200 mm from each other. Filtering compartments are located on the casing of the drum beneath the pulp compartments. The outer rim at the end of the drum includes a valve arrangement through which the filtrate is discharged. The washer may have 3 to 5 stages, in other words the filtrates are led from stage to stage by pumping upstream. The chambers of the washing liquid between different stages are sealed.
Web formation is carried out by feeding pulp into the feed box, the bottom of which is formed of a perforated plate, on which an endless wire cloth is located. The feed box becomes lower towards the washing drum. Liquid is discharged from the pulp in the feed box through the wire cloth and the perforated plate and the pulp is thus thickened on the wire cloth. With the wire cloth moving towards the drum, liquid is continuously discharged from the suspension also due to the pressure caused by the lowered feed box. At the end of the feed box pulp is led to the compartments between the moldings and axial "planks" of length of the drum are thus formed in the compartments. Immediately downstream of the feeding point, the drum has a first washing zone; the apparatus according to said patent publication has five separate zones. A flow of washing liquid is led to each zone, which when pressed through the pulp layer in the compartments of the washing drum displaces the previous liquid there. As mentioned above the filtrates are led upstream from one zone to another. In other words, pure washing liquid is pumped to the last washing zone and the displaced filtrate is led to the second last zone to operate as washing liquid there. Subsequent to the last washing zone the "pulp planks" are removed from the drum, for example, by compressed air blow and are transferred forwards with a screw conveyer.
The specific square capacity of this type of pressure washer when having four stages, is about 2.4 BDMT/m.sup.2 /d. The thickness a "pulp plank" is about 55 mm, and it may reach a consistency of 15 to 17%. The washing water flowing from the compartments, however, dilutes the consistency to 10 to 12%. The consistency of the pulp being fed to the washing drum is 3 to 6%. The rotational speed being used with the drum is about 0.3 rpm.
All said apparatuses, apart from the diffusers are characterized in that the consistency of the pulp being fed to the washer is relatively low, at its maximum 6%. In other words the pulp is to be diluted prior to the washing to less than half of the value of the preceding treating stages, which is 10 to 15%. Thus the amount of liquid in the pulp at least doubles. If it were possible to carry out the washing at high consistency, savings might be gained both in the size of the equipment, in the energy consumption and also in the amount of the filtrate to be led for evaporation. The problem is, however, that there has not been appropriate equipment to feed high consistency, over 6%, pulp to the washer. On the other hand, it is also a known fact that when the pulp thickens the air content of the suspension grows and foam problems arise in the washing. Also other pulp treating devices, such as thickeners, have similar problems.